1. Field of the Invention
The present invention generally relates to communications over packet networks and, more particularly, to supporting text telephone modem communications over packet networks.
2. Background Art
Today, the traditional analog telephony is being rapidly replaced with digital IP (Internet Protocol) telephony, which uses various techniques to break voice into data packets for transmission over packet networks. Analog modems have been utilized to provide data, facsimile, voice and text communications over twisted pair telephone lines for decades. Because a significant infrastructure is in place using such modems, the packet networks need to address and support modem communications.
One type of modem that has been in use for many years is the Text Telephone (TTY) modem, which can be used in an analog text telephone. Due to the recent move to the packet network environment, TTY modems have created a unique challenge for support over the packet networks. TTY modems, are also sometimes referred to as a TDD (Telecommunication Device for the Deaf) modems, which are special devices that allow deaf, hard of hearing, or speech-impaired people to use the telephone to communicate. Telephone communication for such users is accomplished by allowing them to type messages back and forth to one another instead of talking and listening. A TTY modem is required at both ends of the conversation in order to establish a telephone communication. However, TTY modems do not use the same communication protocol as regular data modems and, thus, TTY modems require the packet network gateways to perform a different detection algorithm for detecting TTY modems in order to configure the gateways properly. There are several different TTY modems, such as Baudot @ 45.45 baud (U.S. TTYs); Baudot @ 50 baud (international Baudot used in Australia, England, and other countries); V.21/text telephone version (Sweden, Norway, and Finland); DTMF (Denmark, Holland, other countries); and EDT (“European Deaf Telephone”) (Germany, Austria, Switzerland, and other countries).
FIG. 1 illustrates a block diagram of a conventional communications network 100 utilizing TTY modems for communication over a packet network protocol, such as Internet Protocol, which may also be referred to as Text over Internet Protocol (“ToIP”). As shown, communications network 100 includes first text telephone modem (TTY1) 102 in communication with first gateway device (G1) 104 over first phone line 108, and second gateway device (G2) 114 in communication with second text telephone modem (TTY2) 122 over second phone line 118. Communications network 100 further includes a packet network protocol, such as IP 112 to provide communications between first gateway device 104 and second gateway device 114. IP 112 implements the network layer (layer 3) of a network protocol, which contains a network address and is used to route a message to a different network or subnetwork. IP 112 accepts packets from the layer 4 transport protocol, such as Transmission Control Protocol (“TCP”) or User Data Protocol (“UDP”), and adds its own header and delivers the data to the layer 2 data link protocol. TCP provides transport functions, which ensures that the total amount of bytes sent is received correctly at the other end. UDP, which is part of the TCP/IP suite, is an alternate transport that does not guarantee delivery. UDP is widely used for real-time voice and video transmissions where erroneous packets are not retransmitted.
The communication process begins when TTY1 102 originates a call to TTY2 122. G1 104 receives the call and informs G2 114 of the call for TTY2 122 over communications network 100 and, as a result, G2 114 calls TTY2 122. Typically, in the default mode of operation, G1 104 and G2 114 communicate in voice mode and use compressed voice protocol, such as the ITU standard G.723.1. Conventionally, G1 104 and G2 114 detect modem communications to switch to an uncompressed voice protocol, such as an ITU standard G.711, which provides toll quality audio at 64 Kbps using either A-Law or mu-Law pulse code modulation methods. This uncompressed digital format is used in order to allow easy connections to legacy telephone networks. By switching to G.711, the signals generated by TTY1 102 may propagate through from G1 104 to G2 114 in a more intact manner in order to reach TTY2 122, since an uncompressed format conventionally matches the format used on phone lines 108 and 118, such that signal sample values output from G2 114 from phone line 118 would generally be identical to the corresponding sample values input to G1 104 from phone line 108, thus allowing for the propagation delay through G1 104, IP 112 and G2 114, which is conventionally known as the pass-through mode for modems.
The predominant text telephony modulation schemes use carrier-less modulation. For example, in the U.S., each key depression on the text telephone causes the transmission of a complete Baudot character, including start and stop bits. FIG. 2 illustrates a manual transmission of the text “GA” by typing on the text telephone, which is followed by a “new line” character. As shown, The “G” and “A” characters are spaced apart in time at the rate at which they are typed. The “new line” character is broken into two characters, namely “carriage return” and “line feed”. Therefore, there is no indication prior to receiving the first character, i.e. “A”, by G1 104 from TTY1 102 that the signal is a text telephone signal.
The proper detection of TTY modems in a timely fashion is crucial for switching from one mode of operation by G1 104 and G2 114 to another modem of operation in order to support communications between TTY modems over the packet network. Today, a major implementation issue for ToIP is the potential for either losing or repeating the first character of a TTY modem communication, which is considered regression in functionality vis-à-vis communications over the PSTN (Public Switch Telephone Network). Furthermore, in applications such as PSAPs (Public Safety Answering Points) or emergency services, where reliability is paramount, losing or repeating a character is not a mere inconvenience, but can be life threatening. As a further example, if the first character is part of a password or a user identification, user's login may fail if the first character is lost or repeated due to improper or untimely switching from one mode of operation to another by the gateways.
Accordingly, there is an intense need in the art for a proper detection of TTY modems and timely switching by the gateways to support TTY modem communications over packet networks.